It is the present practice of a Beckwith Electric Company line of generator and transformer protective relays to sample alternating current (AC) voltage and current waves some small number of times, such as 16 per cycle, and then use Fourier transforms to compute phasor quantities representing fundamental components of the waves.
Reference is made to the following book and articles on the present related state of the art:
[1] Phadke, A. G. and J. S. Thorp, Computer Relaying for Power Systems, John Wiley and Sons, New York, 1988, pp. 123-129.
[2] Hector J., Altuve F., Ismael Diaz V. and Ernesto Vazquez M., xe2x80x9cFourier and Walsh digital filtering algorithms for distance protectionxe2x80x9d, IEEE Transactions on Power, vol. 11, No. 1, February 1996, pp. 457-462.
[3] Harlow, J. H., xe2x80x9cA multifunction protective relay for the cogeneration industryxe2x80x9d, IEEE Computer Application in Power, vol.3, No.4, 1990, pp.25-30.
[4] Yalla, Murty V. V. S., xe2x80x9cA digital multifunction protective relayxe2x80x9d, IEEE Transactions on Power Delivery, vol.7, No.1, 1992, pp.193-201.
Making use of inventions as found in U.S. Pat. No. 5,224,011, MULTIFUNCTION PROTECTIVE RELAY, one typical product, model M-4320 manufactured by the Beckwith Electric Company, uses a digital signal processor (DSP) running at some 80 megabits per second, to compute Fourier transforms of the waves as digital samples are taken. The results are fed to a dual ported random access memory where they are available to a host processor. The host processor, operating at 10 megabits per second, uses the results to compute the existence of electrical fault conditions and to output commands for tripping circuit breakers so as to quickly remove electric power feeding faults.
In the M-4320 it is necessary to use analog component anti-aliasing filters to prevent errors due to problems introduced by the use of a small number of digital samples of the AC wave. Since the electronic circuit is connected to earth ground, transformers must be used for all current and voltage inputs.
Another Beckwith Electric Company product line makes use of inventions contained in U.S. Pat. No. 4,310,771 METHODS FOR TRANSFERRING POWER IN A SYNCHRONOUS MODE TO A MOTOR BUS, U.S. Pat. No. 4,256,972 POWER TRANSFER RELAY and U.S. Pat. No. 4,311,919 TRANSFER LOGIC CONTROL CIRCUITRY.
In products based on the above referenced U.S. Pat. Nos. 4,310,771, 4,256,972 and 4,311,919 a first analog device is used to develop information useable for the transfer of a motor bus from a first source to a second source, not necessarily in phase with the first, in a sequence of criteria. A second combined analog and digital device uses the information to effect the transfer. Use of these devices results in a large and expensive combination of equipment.
The transfer of motor busses is sometimes made manually for operating reasons. At other times the transfer may be initiated automatically due to the failure of a bus presently in use.
The first criterion used is called the xe2x80x9cfast transferxe2x80x9d and operates if calculations indicate that the transfer can be completed while the phase angle between the two sources is within a selected phase angle window. The second criterion first separates the motors from the first source and then makes a synchronous transfer when the isolated motor bus falls behind the second source by 360 degrees. The third criterion of transfer is used if the motor bus voltage falls below a selected value before the 360 degree point is reached.
While a bus transfer device does not have the requirement to trip a circuit breaker in order to interrupt a fault current, it is classified herein as a protective device since proper operation does protect equipment from damage. In addition, as in protective relays that detect fault conditions, its operation likewise depends on a cycle by cycle measurement of voltage and current waves.
In making improvements over the present art, use is made herein of the following inventions, all by the present inventor, Robert W. Beckwith.
A) U.S. Pat. No. 5,315,527, METHOD AND APPARATUS PROVIDING HALF-CYCLE DIGITIZATION OF AC SIGNALS BY AN ANALOG-TO-DIGITAL CONVERTER, describes apparatus and methods for sensing positive half cycles of AC signals.
B) U.S. Pat. No. 5,544,064, APPARATUS AND METHOD FOR SAMPLING SIGNALS SYNCHRONOUS WITH ANALOG-TO-DIGITAL CONVERTER, describes apparatus and methods useful for obtaining digital samples of AC waves synchronous with free running analog to digital converters (ADCs).
C) U.S. Pat. No. 5,774,366, A METHOD FOR OBTAINING THE FUNDAMENTAL AND ODD HARMONIC COMPONENTS OF AC SIGNALS, describes methods for obtaining the fundamental component and odd harmonics of half wave AC signals.
D) U.S. patent application Ser. No. 710,816, TWO WAY PACKET RADIO INCLUDING SMART DATA BUFFER AND PACKET RATE CONVERSION, the inventer herein, filed on Sep. 23, 1996 describes apparatus and methods of communicating synchronously with the power frequency as useful in the present invention.
The present invention combines use of the half wave technology of U.S. Pat. No. 5,315,527 together with the synchronous linear technology of U.S. Pat. No. 5,544,064 and methods for obtaining the fundamental component and odd harmonics of a half wave AC signal of U.S. Pat. No. 5,774,366 in reducing the hardware and software requirements while at the same time increasing the operating speed of devices useful in AC electric power apparatus. Means of communicating between devices using the present invention and any of the many protocols in use at the time of writing of this application are provided by the inventive means and methods by reference U.S. patent application Ser. No. 710,816.
U.S. Pat. No""s. 5,315,527, 5,544,064, 5,774,366 and U.S. patent application Ser. No. 710,816 are incorporated herein by reference.
This invention utilizes tables of sine and cosine functions multiplied by digital samples of AC waves to obtain, by cross correlation, the values of phasors for the waves, thus eliminating the need for Fourier transform calculations required in many prior art devices. These phasors are obtained one or more times per cycle of AC voltage and current waves in order to determine output requirements, generally in the form of operation of solid state switches.
In a first microprocessor, linear programs, using fixed operating time loops, operate synchronously with microprocessor ADCs. These ADCs produce digital samples in matching loops and require no ongoing instructions from the linear programs. A second microprocessor obtains the phasor information via a high speed synchronous data bus from the first microprocessor and computes results such as fault clearing and motor bus transfer conditions. Use of linear programs loops matching those of the first microprocessor avoids data clashes between the two microprocessors. Both microprocessor programs run continuously without the use of interrupts.
Functions of the two microprocessors are combined into one microprocessor for protection of single phase equipment and for motor bus transfer.
Examples are given for use of the invention.